Why the date of Diwali changes every year
Diwali follows the moon rather than the Gregorian calendar, so the festival lands on a different date each year.
Diwali is fixed to the new moon of the Hindu month of Kartik, the darkest night of the lunar month. Because Hindu months are lunar and about 11 days shorter than a solar year, the festival drifts against the Gregorian calendar unless the lunar calendar is corrected. About once every three years an extra month, Adhik Maas, is inserted to pull the calendar back in line with the seasons. In 2026 an extra Jyeshtha month ran from May 17 to June 15, which pushed Diwali from October 20 last year to November 8 this year. The festival is expected to fall on October 29 in 2027. Almanacs decide the exact day by checking when the new-moon tithi is in effect during the early evening, when the main rituals take place, so the date can vary between regions when the tithi covers both days.